Electrophotographic printers and copiers commonly use composite photoconductors as the means to create latent electrostatic images. Composite photoconductors have a photosensitive layer (charge generation layer or CGL) and a charge transport layer (CTL). The CGL is able to generate a hole/electron pair and can transport charges of both polarities. The CTL layer is normally designed such that it will only transport one charge species (only negative or only positive charge carriers) to provide stability. Due to the fact that the CTL can transport only one species of charge carrier, a composite design results in a photoconductor that is meant to be useful for creating latent images when the surface is charged in the intended polarity. If the photoconductor is charged in the opposite polarity and then exposed to light, charges may become trapped in the photoconductor structure. These trapped charges result in poor performance or permanent damage of the photoconductor.
It is common in electrophotographic systems to have functions that require the surface of the photoconductor to be charged in the polarity opposite to that with which they are designed to be used. One such function is the corona charger that conditions the photoconductor prior to the photoconductor cleaning apparatus. The charger must charge the photoconductor and the residual toner left on the photoconductive member after image transfer to a level to facilitate removal of the residual toner. Another example of a function that requires the use of a charger of the opposite polarity is the transfer charger in a Discharge Area Development (DAD) device. DAD electrophotographic printers make use of toners that charge in the same polarity in which the photoconductive member is light sensitive. A charger of opposite polarity is required in the transfer function to cause the toned image to migrate to the receiver member.
To simplify the description of this invention, we will assume that the composite photoconductor in use is designed with a hole transport CTL. In this case, the film is designed to be charged negatively. A corona charger operating in the positive mode with such a photoconductor would benefit from use of this invention. The discussion could be applied to a positive charging photoconductor by changing all the references to negative charging to positive charging and visa versa.
Corona chargers in electrophotographic devices need to be spaced accurately from the photoconductive member to assure that uniform charge is applied to the photoconductor surface. It is common to have an adjustment feature designed into the corona charger to facilitate setting up a precise spacing between the charger and the surface of the photoconductive member. The uniformity of the charge laid down on the photoconductive member is measured by the use of an electrostatic voltmeter. The spacing of the corona charger to the photoconductive member is varied until a uniform charge is obtained. To assure consistent charging of the photoconductive member, it is required that the charge is erased after being measured by the electrostatic voltmeter. The use of erasure assures that a consistent charge level is on the photoconductive member as it enters the corona charger that is being set up.
The setup of a corona charger which charges the photoconductor in the negative polarity is straight forward. The photoconductor is charged by the corona charger to be adjusted, measured by an electrostatic voltmeter, and then erased by an exposure device to return the film to a stable level. Setup of chargers in an electrophotographic system which charges the photoconductive member in the positive polarity is more difficult. This is due to the fact that if the photoconductor is charged in the positive polarity the charge cannot be erased by an exposure device.
In systems that require the setup of a charger that operates in the positive polarity, use of an additional charging device which charges the photoconductive member to the negative polarity is commonly used. Normally the primary charger is used to accomplish this function. This charger reverses the charge level on the photoconductive member, allowing the photoconductor to be erased with an exposure device without damaging the photoconductor from trapped charge in the photoconductive layer or layers. This provides a stable input voltage for the positive charger being adjusted for uniformity.
The use of this method has some drawbacks. This method requires that either:
1. The electrostatic voltmeter probe be placed between the charger being setup and the charger that returns the photoconductive member to the negative polarity. This is often inconvenient and results in the need to facilitate multiple positions to locate the electrostatic voltmeter probe depending on the charger being setup, or PA1 2. An algorithm be provided that cycles the negative charger, that returns the photoconductive member to the negative polarity, off and on precisely. The readings for the positive charger being set up in the positive polarity are taken for one revolution, then the photoconductive member is charged to the negative polarity by the negative charger and erased. The cycle can then begin again.